It starts with a certain angle: a smartphone tilted at 45 degrees just above your eyeline is generally deemed the most forgiving. Then a light source: the flattering beam of a backlit window or a bursting supernova of flash reflected in a bathroom mirror, as preparations are under way for a night out.

The pose is important. Knowing self-awareness is conveyed by the slight raise of an eyebrow, the sideways smile that says you're not taking it too seriously. A doe-eyed stare and mussed-up hair denotes natural beauty, as if you've just woken up and can't help looking like this. Sexiness is suggested by sucked-in cheeks, pouting lips, a nonchalant cock of the head and a hint of bare flesh just below the clavicle. Snap!

Afterwards, a flattering filter is applied. Outlines are blurred, colours are softened, a sepia tint soaks through to imply a simpler era of vinyl records and VW camper vans.

All of this is the work of an instant. Then, with a single tap, you are ready to upload: to Twitter, to Facebook, to Instagram, each likeness accompanied by a self-referential hashtag. Your image is retweeted and tagged and shared. Your screen fills with thumbs-up signs and heart-shaped emoticons. You are "liked" several times over. You feel a shiver of – what, exactly? Approbation? Reassurance? Existential calm? Whatever it is, it's addictive. Soon, you repeat the whole process, trying out a different pose. Again and again, you offer yourself up for public consumption.

This, then, is the selfie: the self-portrait of the digital age. We are all at it. Just type "selfie" into the Twitter search bar. Or take a look at Instagram, where over 90m photos are currently posted with the hashtag #me.

Adolescent pop poppet Justin Bieber constantly Tweets photos of himself with his shirt off to the shrieking delight of his huge online following. Rihanna has treated her fans to Instagrammed selfies of her enjoying the view at a strip club, of her buttocks barely concealed by a tiny denim thong and of her posing with two oversize cannabis joints while in Amsterdam. Reality TV star Kim Kardashian overshares to the extent that, in March, she posted a picture of her own face covered in blood after undergoing a so-called "vampire facial". In the same month, the selfie-obsessed model and actress Kelly Brook banned herself from posting any more of them (her willpower lasted two hours).

A 'selfie' by Kelly Brook. Public domain

The political classes have started doing it too. President Obama's daughters, Sasha and Malia, took selfies at his second inauguration. In June, Hillary Clinton got in on the act after her daughter, Chelsea, tweeted a joint picture of them taken on her phone at arm's length. Earlier this month, three sisters from Nebraska stormed the field of a college baseball match and filmed themselves while doing so, eventually being removed by security guards. Stills from the six-second Vine video clip became known as "the most expensive selfie of all time" after it emerged that the sisters were facing a $1,500 fine.

The trend has even reached outer space: in December, Japanese astronaut Aki Hoshide took what might be the greatest selfie of all time at the International Space Station. The resulting image encompassed the sun, the Earth, two portions of a robotic arm, a spacesuit and the deep darkness of the infinite beyond.

"The selfie is revolutionising how we gather autobiographical information about ourselves and our friends," says Dr Mariann Hardey, a lecturer in marketing at Durham University who specialises in digital social networks. "It's about continuously rewriting yourself. It's an extension of our natural construction of self. It's about presenting yourself in the best way … [similar to] when women put on makeup or men who bodybuild to look a certain way: it's an aspect of performance that's about knowing yourself and being vulnerable."

Although photographic self-portraits have been around since 1839, when daguerreotype pioneer Robert Cornelius took a picture of himself outside his family's store in Philadelphia (whether he had the help of an assistant is not known), it was not until the invention of the compact digital camera that the selfie boomed in popularity. There was some experimentation with the selfie in the 1970s – most notably by Andy Warhol – when the Polaroid camera came of age and freed amateur photographers from the tyranny of the darkroom. But film was expensive and it wasn't until the advent of digital that photographs became truly instantaneous.

The fact that we no longer had to traipse to our local chemist to develop a roll of holiday snaps encouraged us to experiment – after all, on a digital camera, the image could be easily deleted if we didn't like the results. A selfie could be done with the timer button or simply by holding the camera at arm's length, if you didn't mind the looming tunnel of flesh dog-earing one corner of the image.

As a result, images tagged as #selfie began appearing on the photo-sharing website Flickr as early as 2004. But it was the introduction of smartphones – most crucially the iPhone 4, which came along in 2010 with a front-facing camera – that made the selfie go viral. According to the latest annual Ofcom communications report, 60% of UK mobile phone users now own a smartphone and a recent survey of more than 800 teenagers by the Pew Research Centre in America found that 91% posted photos of themselves online – up from 79% in 2006.

"A lot of the cameras on smartphones are incredibly good," says Michael Pritchard, the director general of the Royal Photographic Society. "The rise of digital cameras and the iPhone coincided with the fact that there are a lot more single people around [than before]. The number of single-occupancy households is rising, more people are divorcing and living single lives and people go on holiday by themselves more and don't have anyone else to take the picture. That's one reason I take selfies: because I do actually want to record where I am."

But if selfies are simply an exercise in recording private memories and charting the course of our lives, then why do we feel such a pressing need to share them with hundreds and thousands of friends and strangers online? To some, the selfie has become the ultimate symbol of the narcissistic age. Its instantaneous nature encourages superficiality – or so the argument goes. One of the possible side-effects has been that we care more than ever before about how we appear and, as a consequence, social acceptance comes only when the outside world accepts the way we look, rather than endorsing the work we do or the way we behave off-camera.

The American writer John Paul Titlow has described selfie-sharing as: "a high school popularity contest on digital steroids". In an article published on the website ReadWrite earlier this year, Titlow argued that selfie users "are seeking some kind of approval from their peers and the larger community, which thanks to the internet is now effectively infinite".

Indeed, although many people who post pictures of themselves on the internet do so in the belief that it will only ever be seen by their group of friends on any given social network, the truth is that the images can be viewed and used by other agencies. There are now entire porn sites devoted to the "amateur" naked selfie and concerns have recently been raised that jilted lovers can seek their revenge by making explicit images of their ex publicly available online.

Aki Hoshide's outer space selfie. Photograph: AP

The preponderance of young women posing for selfies in a state of undress is a potentially worrying issue. When the model Cara Delevingne Instagrammed a picture of her nipples poking through a black lace top, it rapidly got over 60,000 "Likes".

Dines argues that although men can "gain visibility" in a variety of ways, for women the predominant way to get attention is "fuckability". And it is true that a lot of female selfie aficionados take their visual vernacular directly from pornography (unwittingly or otherwise): the pouting mouth, the pressed-together cleavage, the rumpled bedclothes in the background hinting at opportunity.

But Rebecca Brown, a 23-year-old graduate trainee from Birmingham, believes her penchant for selfies is neither degrading nor narcissistic. Instead, she explains, it is a simple means of self-exploration.

"It's almost like a visual diary," she says. "I can look back and see what I looked like at a particular time, what I was wearing. It's exploring your identity in digital form. To me it's not about nudity or having a raunchy or raw kind of look… People think if you take pictures of yourself, you're self-obsessed but that's like saying if you write a diary or an autobiography, you're self-obsessed. Not necessarily. A selfie is a format and a platform to share who you are."

Does she feed off the social approval that a selfie can generate?

"I suppose you take photos to see what you look like," Brown concedes. "Before I go out, I'll take a couple of pictures almost to see how I look in other people's eyes. In the same way that if you wrote a really good piece of work and had people commenting about how good it was, or if you put something on Twitter that people retweeted, if people start liking your selfie, then obviously you're going to get a natural buzz. It gives you a nice boost and you can walk with that little bit more confidence."

There is nothing new about this, of course. Human beings are social animals and have long been driven by the need for approval and self-affirmation – albeit on a smaller scale. The desire for a pictorial representation of the self goes all the way back to early handprint paintings on cave walls more than 4,000 years ago. In a fast-paced world of ever-changing technology, it could be argued that the selfie is simply a natural evolution of those hands dipped in paint.

"As with so many 'new trends', this one has a fairly distinguished prehistory," explains essayist and author Geoff Dyer. "In 1925 DH Lawrence was bemoaning the way that 'each of us has a complete Kodak idea of himself'. This new phenomenon of the selfie has already been turned into a work of art which is also a sort of visual essay: Richard Misrach's 11.21.11 5.40pm consists of him taking a telephoto shot of a couple on a beach taking a picture of the sea. Then we zoom in closer and closer on each subsequent page until we are able to see the screen of their phone in which is revealed… a self-portrait."

The popularity of the selfie is, says Mariann Hardey, "an extension of how we live and learn about each other" and a way of imparting necessary information about who we are. By way of an example, Hardey says that when her father died suddenly last year, she took refuge in her Instagram feed.

"I couldn't bear the conversations but one way to prove to friends that I was OK was to take a picture of myself," she says. "That revealed something very important to my friends – one, that I was still functioning and, two, I was out doing stuff. An image can convey more than words."

The idea that young women are self-objectifying by posing semi-pornographically for selfies is, she believes, a dangerous one.

"When we're talking about what is acceptable for women in terms of constructing an image, we need to be very careful of not heading down into the territory of 'she was wearing a short skirt, so she was asking to be raped'. We should avoid that argument because it's probably an extension of more patriarchal demands.

A 'selfie' by Hillary and Chelsea Clinton. Photograph: Chelsea Clinton/AP

"Women should be allowed to portray themselves in a way they feel enhanced by. Who didn't experiment with cutting their hair off and dying it pink when they were younger? This is just a natural progression of experimenting with the changing interfaces of being young and one of these interfaces, yes, is sexual identity."

A selfie can, in some respects, be a more authentic representation of beauty than other media images. In an article for Psychology Today published earlier this year, Sarah J Gervais, an assistant professor of psychology, wrote that: "Instagram (and other social media) has allowed the public to reclaim photography as a source of empowerment… [it] offers a quiet resistance to the barrage of perfect images that we face each day. Rather than being bombarded with those creations… we can look through our Instagram feed and see images of real people – with beautiful diversity.

"Instagram also allows us the opportunity to see below the surface. We capture a glimpse into the makings of people's daily lives. We get a sense of those things that make the everyday extraordinary."

The appeal for celebrities like Bieber, Kardashian et al is connected to this. The expansion of social networking has enabled them to communicate directly with their fanbase and to build up large, loyal followings among people who believe they are getting a real glimpse into the lives of the rich and famous.

"If you're going for a younger audience, you're expected to engage with every media channel available to you," says leading PR Mark Borkowski. "Every aspect of Rihanna's life is about her letting people in. Some people are very natural and normal about it and completely comfortable with being 'on' and that's fine. But it becomes unstuck if it's not real. A selfie has to be 'the real you'. It works if you can give people a manageable piece of reality which is who you are."

The key is the idea of "manageable reality": celebrities can now exercise more control than ever over the dissemination of their image. The paradox at the heart of the selfie is that it masquerades as a "candid" shot, taken without access to airbrushing or post-production, but in fact, a carefully posed selfie, edited with all the right filters, is a far more appealing prospect than a snatched paparazzo shot taken from a deliberately unflattering angle.

"It's about self-exposure and control," says artist Simon Foxall, whose work questions the parameters of individuality and self-expression. "A selfie blurs the line between 'reality' and the performance of a fantasy self, so one collapses into the other."

Beyond that, a judicious use of selfies can make good business sense too: Alexa Chung and Florence Welch have both used selfies to post daily updates on what they are wearing, thereby cementing their position as modern style icons and guaranteeing, no doubt, the continuation of a series of lucrative fashion deals. (Chung, for one, has designed a womenswear line for the fashion brand Madewell for the last three years.)

The website What I Wore Today began as a site that featured young entrepreneur Poppy Dinsey posing for a daily selfie, in a different outfit for every day of the year. It became an internet hit and has now expanded to allow users to upload their own images, as well as generating advertising revenue by featuring online links to clothing retailers.

Poppy Dinsey began her fashion blog by using selfies. Linda Nylind/Guardian

"People like the control selfies give them," says Dinsey (pictured below). "Sometimes it's just a practical matter of not having anyone around to shoot you and that's why I always took my own pictures in mirrors for WIWT. But you're deciding how to frame yourself – you're not trusting someone else to make you look good. With front-facing cameras on iPhones, and so on, you can see the picture you're taking and frame it perfectly to show yourself off as best as possible – your mate isn't going to make the same effort when taking your picture. Plus, you can retake and retake without anyone having to know how much vanity has gone into that 'casual' pose."

In some ways, of course, the notion of control is disingenuous: once a selfie is posted online, it is out there for public delectation. Future employers can see it. Marketers can use it. A resentful former lover could exploit it.

You can use digital technology to manipulate your own image as much as you like. But the truth about selfies is that once they are online, you can never control how other people see you.